This invention relates generally to overcurrent tripping devices, and more particularly, to a digital overcurrent tripping arrangement which contains a volatile digital memory for the thermal excitation state, and a microprocessor having an analog-to-digital converter.
It is generally known that the instantaneous thermal state prevailing in a system which has been disconnected by overcurrent tripping devices must be taken into consideration by the reconnecting facility. The thermal state prior to reconnection depends not only upon the state which existed during the tripping, but also on the time which has passed since the tripping during which time the disconnected system has cooled.
In known overcurrent tripping devices which operate thermally, the information relevant to the thermal tripping is stored in the form of heat. After responding, such overcurrent tripping devices cool off so that the cooling of the system after tripping can be taken into consideration automatically.
In known digitally operated overcurrent tripping devices on the other hand, a purely mathematical simulation of the thermal excitation state is performed in a microprocessor, where the numerical storage of this state occurs in a volatile memory, i.e., not permanent. If, as is done in practical systems, the digital overcurrent tripping device takes its supply energy from the network to be monitored, the digital information which is stored in the volatile memory disappears if the overcurrent tripping device responds, or the network to be monitored is otherwise switched off.
It is, therefore, an object of this invention to provide a digital overcurrent tripping arrangement wherein numerical values which correspond to a thermal excitation state are preserved when energy to the tripping arrangement is discontinued.
It is a further object of this invention to provide a digital overcurrent tripping arrangement wherein a cooling-down characteristic of the system is simulated.